Such lock and key systems, lock units and associated interchangeable cylinder lock cores are frequently used, especially in the US, in locking systems with a large number of lock units serving as door locks. From time to time, many of these cylinder lock cores need to be replaced quickly whenever an associated key is lost, or a new tenant, owner or user wishes to upgrade the door lock with a new cylinder lock core.
Since the 1930's the Best Lock Company has dominated the US market for this kind of locking systems. Accordingly, there exist today a huge number of such door lock units. A number of manufacturers are providing their own interchangeable cylinder lock cores to replace the existing ones.
However, in general, there are no really high security interchangeable lock cylinder cores on the market.
One reason for this is that the key plug has a relatively small diameter and that most of the space is occupied by cavities for accommodating the locking pins, a keyway as well as the two prong holes in the back region of the key plug. There is no room for a conventional side bar or other kinds of side tumbler mechanisms which normally require a longitudinal extension along the full length of the key plug.
Another reason is that the two prong holes have to be dimensioned so that even a relatively high torque can be transferred to the lock unit by way of a cam or some other locking device coupled to the lock unit, e.g. for closing a door effectively. Therefore, the cam or other locking device has to be securely fastened to each of the prongs, and the prongs have to fit well into the prong holes. They must not lose their engagement or break.
Also, in order to enable an easy mounting of the cylinder lock core into the housing of the existing lock unit, there must be a certain play between the prong holes and the prongs, so as to enable an easy and effective guiding of the prongs into the prong holes. Once, mounted, the engagement must be secure, with no risk for disengagement or breakage.
Accordingly, a good guidance and an effective engagement between the prongs and the prong holes will require that the two prong holes in the back region of the key plug are relatively long, preferably having mutually the same length, so that they will symmetrically carry the same load when a torque is applied.
A recently developed system, including an interchangeable cylinder lock core of the kind defined in the first paragraph above is disclosed in U.S. Pat. No. 8,186,194 (Field et al). In this prior art system, the lock comprises a conventional cylinder lock with a shell and a key plug having two long prong holes in a back region and an auxiliary side locking system which is separate from the normal central row of locking pins. The auxiliary side locking system comprises an auxiliary locking pin being disposed in an associated hole in a side region of the key plug and being movable between a first position, in which a portion thereof projects outside the key plug, and a second position in which the auxiliary locking pin is retracted into said hole. A slider arranged in the key plug, underneath the auxiliary locking pin, is movable axially along the key plug between a first position, where it holds the auxiliary pin in said first position, and a second position, where it permits the auxiliary pin to fall down to its second, retracted position. In this way, the key plug is released so that it may be rotated within the shell. In an alternative embodiment, the auxiliary pin engages directly with a retainer sleeve which surrounds the key plug.
The auxiliary side locking system will ensure that the key plug is effectively locked in its rotary position and cannot rotate as long as the slider is held in its first position. Even if all the regular locking pins are manipulated or “bumped” into their correct releasing positions in an attempt to pick the lock, the remaining auxiliary side locking pin will prevent rotation of the key plug. So, this prior art system will substantially increase the safety against lock picking.
In another prior art exchangeable lock construction of similar design, disclosed in the U.S. Pat. No. 6,981,396 (Kim), there is a key plug with a side locking mechanism including a sidebar which interacts with a number of side locking tumblers. The side bar also interacts with a retainer sleeve (a “tubular operation body”) which extends along the full length of the key plug. Accordingly, on the side provided with the side bar, the key plug will be solid virtually all the way to its rear end, and the associated prong in the associated lock unit will have to be sheared in order to accommodate the key plug and its security full length sidebar. With this construction, the torque to be transferred to the lock unit, by rotating the key plug by means of an inserted key, will be effected by means of one prong only, situated on the opposite side of the key plug.